1. Field of the Invention
The present invention relates to an ink jet printer. The ink jet printer of the present invention includes all devices for printing words, images, etc. by discharging ink towards a print medium. For example, the ink jet printer of the present invention includes copying machines, fax machines, multifunctional products, etc.
2. Description of the Related Art
An ink jet printer has an ink jet head. Usually, the ink jet head comprises an ink passage body and an actuator. The ink passage body comprises a nozzle, an ink chamber, and a pressure chamber. The nozzle discharges ink toward a print medium. The ink chamber houses ink, and the ink chamber and the nozzle communicate. The pressure chamber is disposed between the nozzle and the ink chamber. The actuator comprises a piezoelectric element facing the pressure chamber. There is a piezoelectric element of the following type: the piezoelectric element comprises a piezoelectric layer, a first electrode connected with a front face of the piezoelectric layer, a second electrode connected with a back face of the piezoelectric layer, and a middle layer located between the second electrode and the ink passage body. When an electric potential difference is applied between the first electrode and the second electrode, the piezoelectric layer may contract in a planar direction. The first electrode, the second electrode, and the middle layer cannot contract in the planar direction. As a result, the force for causing the piezoelectric layer to contract in the planar direction is converted into force that bends the entire piezoelectric element in its direction of thickness. Therefore, the piezoelectric element may protrude toward the pressure chamber when the electric potential difference is applied between the first electrode and the second electrode. The capacity of the pressure chamber is reduced when the piezoelectric element protrudes toward the pressure chamber. The pressure of the ink within the pressure chamber is thus increased, and the ink is discharged from the nozzle. When the electric potential difference between the first electrode and the second electrode is cancelled, the state in which the piezoelectric element was protruding toward the pressure chamber is released. The capacity within the pressure chamber is thus increased, and ink is drawn from the ink chamber into the pressure chamber.
When the middle layer is present between the second electrode and the ink passage body, there is a greater amount of transformation in the direction of thickness of the piezoelectric element. Usually, an insulator is utilized in this middle layer. With this configuration, pressure within the pressure chamber may be efficiently increased and decreased. An ink jet printer having the aforementioned configuration is taught in U.S. Pat. No. 6,672,715.
If a print medium (printing paper for example) is charged, an electric charge may be conveyed from the print medium to the ink passage body. The ink passage body may thus be charged, and the electric potential of the ink passage body may become greater than the electric potential of the second electrode. In this case, the components of the ink (mainly hydrogen ions) within the ink passage body are attracted towards the actuator (the second electrode). The components of the ink may enter the actuator, and if hydrogen ions enter the actuator, hydrogen gas may be formed within the actuator. If hydrogen gas is formed within the actuator, the layers within the actuator (e.g. the piezoelectric layer and the second electrode) may separate.
The present invention sets forth a technique capable of preventing the components of the ink within the ink passage body from entering the actuator.